New superconductive material for long-distance energy transmission

In 2015 solar and wind power supplied an average of 33% of Germany’s electricity. Renewables were taking a steadily larger share across Europe, and the German nuclear phase-out, part of the country’s Energiewende, meant the national grid would have to grow.

Renewable power depends on the local weather, on the ability to shift electricity from one region to another, and on building a new long-distance network of lines between European countries.

The options on the table

With existing technology, high-voltage DC overhead lines are the cheapest option, but rural residents and environmental groups strongly oppose their tall pylons.

The alternative is to bury copper or aluminium high-voltage lines, as is done in built-up areas. Over long distances that is unlikely, because it costs much more than an overhead line.

Copper and aluminium lines have another drawback: losses from the electrical resistance of the wire, which heats up and wastes energy. On long lines the loss can reach 10% of the energy transmitted, which across Europe equals the output of 3 to 5 large power stations.

A century of superconductors

In 1911 the Dutch physicist Heike Kamerlingh Onnes discovered superconductivity: some metals cooled to temperatures close to absolute zero lose all electrical resistance. A current in a closed loop of niobium held near absolute zero will flow indefinitely. Kamerlingh Onnes suggested that superconducting wires could be used to carry energy.

Decades later, the American physicist Richard Garwin made the first practical proposal. He calculated that a 1,000 km line carrying 100 GW, then the whole electricity output of the United States, could run through a single underground superconducting cable just 30 cm wide including its cooling. The catch was that cooling the wire to a few degrees above absolute zero would have cost too much.

In the 1980s scientists found ceramic materials that become superconducting at much higher temperatures, up to 70 degrees above absolute zero. Cooling them is cheaper, but these high-temperature superconductors are hard to make and too expensive for long distances, and utilities remained sceptical about them for energy transport.

Magnesium diboride

In 2001 researchers in Japan found that a simple compound, magnesium diboride (MgB2), becomes superconducting at 39 degrees above absolute zero (39 K). In 2011, at the Institute for Advanced Sustainability Studies (IASS) in Potsdam, Germany, the Nobel physics laureate Carlo Rubbia set up a research cluster on superconducting lines for energy transport.

Magnesium diboride exists only as a powder, but researchers at Columbus Superconductors in Genoa, Italy, found they could make long strands by filling copper or nickel tubes with it and sintering them. “High costs barred the way for the energy transportation over long distances with superconductors. MgB2 really changed the game,” said Giovanni Grasso, director general and co-founder of the company.

In 2014 a first test of the MgB2 wire at CERN confirmed that the material was a good choice. “The MgB2 wire was tested with a very high current, up to 20,000 amperes, and this experiment confirmed the potential of MgB2 as the superconductor of choice for long-distance energy transport,” said Adela Marian, a physicist at the IASS.

The planned test

The IASS joined a study on superconductors in the European project BEST PATHS, together with CERN, Columbus and other research groups. They planned a demonstration testing an MgB2 cable under conditions matching future transmission systems.

At 200 to 320 kV and a DC current of up to 10,000 amperes, the test was to show 3.2 GW, the output of three large power stations, passing through a superconducting cable 12.5 mm across. The cable would sit in a tube with its cooling system, holding it at an optimal 20 K.

The cooling design was still being worked out. It would run in two stages: an outer circuit using nitrogen would bring the temperature down to 70 K, then the cable itself would be cooled to 20 K, with either liquid hydrogen or helium gas as the coolant.

“The cryogenic system, on which we still have to work, is the main aspect still limiting this application,” said Christian-Eric Bruzek, project leader at Nexans, the French company in the project that assembled the strands produced by Columbus.

The verdict was still open. In the long run, Marian said, underground MgB2 cables were expected to compete even with overhead lines.

The test did go ahead: the finished cable system, rated for up to 3.2 GW, was presented in La Spezia in July 2018, as told in the report on the record-setting superconducting cable.